Tonight we will have a battle between the Restless Dead and Beastmen Raiders with the Mark of Shornaal. If a Restless Dead warband has two scarecrows and the Shaman for the Beastmen Raiders casts Shornaal's Temptation on the Necromancer in the Restless Dead then will the Beastmen Raiders also take control of the Scarecrow that is controlled by the Necromancer? It makes sense but would be a dreadful double-blow to the Restless Dead.

Going further down the rabbit hole if there is a battle between two warbands with the Mark of Shornaal and a third warband then if one Seer/Shaman uses Shornaal's Temptation on the other Seer/Shaman who has already used Shornaal's Temptation on a warrior from the third warband then would the first warband get control of both the Seer/Shaman and the warrior from the other warband? I doubt this particular scenario will ever occur but the scenario in my first question could very well occur tonight.

This is my suggestion on how to rule this, taking into account both fluff and balance aspects:

Scarecrows & Shornaal's TemptationA Necromancer of the Restless Dead warband failing his Leadership test for the Shornaal's Temptation spell is then controlled by the Shornaal spell-caster. This will immediately break the bond with any Scarecrow that Necromancer controls and the Scarecrow is imediately taken out of action. Note that the Liche is immune to psychology and thus automatically passes Shornaal's Temptation.

Chained Shornaal's TemptationsSeer1 controls warriorX via the Shornaal's Temptation spell. Now Seer2 successfully casts Shornaal's Temptation on Seer1. This breaks the bond between Seer1 and warriorX. Seer1 is now controlled by the player of Seer2 and warriorX is controlled by its original player again. The Seers' casting these spells do not have the magical powers to control many persons at a time, but more powerful characters may very well be able to do so...